Orthopaedic implants have evolved into many types of devices to assist in arthrodesis and correction of bone defects of a congenital, degenerative, or trauma related nature. Among the various types of orthopaedic implants are plate type devices. Plate type devices, like most devices, with the exception of endoprotheses, are temporary devices attached to stabilize two bone fragments or two bones, such as vertebra, until healing of the fragments or fusion of the two bones has occurred.
These devices are designed to be load sharing rather than load bearing. Load bearing devices typically carry all or bear all the stress. This is sometimes referred to as "stress shielding". Load sharing transfers some amount of the stress from the device to the bone itself. This transfer of load to the bone causes stress and this stress becomes the mechanism that triggers the body to start the healing or fusion process.
Some applications require different types of fastener devices, such as screws, pins, staples, or cerclage wire, in conjunction with the plate devices to secure them to the bone to provide the required stabilization. Many fasteners are designed specifically for the two different types of bone within the body.
The two types of bone are cortical and cancellous bone. Cortical bone is typically the hard, dense shell of the bone that provides the structural strength. Cancellous bone is the more spongy and soft bone located inside the cortical shell as part of the marrow of the bone, which provides the blood supply and nutrients for the bone.
Due to the hard, dense shell cortical bone is typically more stable for the placement of screws for fixation. Cancellous bone is weaker compared to cortical bone. Screws designed for fixation in cortical bone are typically placed through one cortex, through the cancellous or marrow, and into the far cortex of the bone. This is referred to as bicortical screw fixation. Screws designed for cancellous bone are typically designed with a buttress type thread to be able to put as much material as possible between successive threads to increase the shear area in the cancellous bone. Cancellous screws are typically placed through one cortical wall and sized in length such that the end or tip of the screw does not encounter the cortex on the far side of the bone but ends in the cancellous structure. Cancellous screws may be used instead of cortical screws when penetrating the far cortex is not preferable. In some cases, penetrating the far cortex may result in damage to arterial or neurological structures. However, one concern of screws placed unicortically into cancellous bone can be the tendency for the screw to "back out" from the plate device under cyclic loading and/or osteoporotic conditions or due to poor quality of the bone.
Devices used in applications involving the fusion or arthrodesis of two bones, such as the vertebra, require the cartilaginous material to be removed between them and the bone surface abraided to encourage a bleeding surface. Blood supply from the bleeding surfaces are required in order for the bone to fuse. Fusion of a joint involves removing the cartilaginous material in the joint and requires the cartilaginous surface of the articular joint to be abraided to encourage a bleeding surface for fusion. Fixation and stabilization must be adequate for the time required for a fracture to heal or two bones to fuse.
In certain applications where the devices are used in close proximity to a joint, the device should be designed such that it does not cause damage or have adverse effects to the articular surfaces of the joint. Further considerations of implant design should also be given to ensure that ligaments and tendon structures, usually located close to the joint, that come into contact with the implant are not compromised in any way by excess material, rough surfaces, or sharp edges. The profile of the present plate construct has been designed with these considerations.